The present invention relates to polypropylene fibres and to fabrics produced from polypropylene fibres.
Polypropylene is well known for the manufacture of fibres, particularly for manufacturing non-woven fabrics.
EP-A-0789096 and its corresponding WO-A-97/29225 discloses such polypropylene fibres which are made of a blend of syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP). That specification discloses that by blending from 0.3 to 3% by weight of sPP, based on the total polypropylene, to form a blend of iPP-sPP, the fibres have increased natural bulk and smoothness, and non-woven fabrics produced from the fibres have an improved softness. Moreover, that specification discloses that such a blend lowers the thermal bonding temperature of the fibres. Thermal bonding is employed to produce the non-woven fabrics from the polypropylene fibres. The specification discloses that the isotactic polypropylene comprises a homopolymer formed by the polymerisation of propylene by Ziegler-Natta catalysis. The isotactic polypropylene typically has a weight average molecular weight Mw of from 100,000 to 4,000,000 and a number average molecular weight Mn of from 40,000 to 100,000, with a melting point of from about 159 to 169xc2x0 C. However, the polypropylene fibres produced in accordance with this specification suffer from the technical problem that the isotactic polypropylene, being made using a Ziegler-Natta catalyst, does not have particularly high mechanical properties, particularly tenacity.
WO-A-96/23095 discloses a method for providing a non-woven fabric with a wide bonding window in which the non-woven fabric is formed from fibres of a thermoplastic polymer blend including from 0.5 to 25 wt % of syndiotactic polypropylene. The syndiotactic polypropylene may be blended with a variety of different polymers, including isotactic polypropylene. The specification includes a number of examples in which various mixtures of syndiotactic polypropylene with isotactic polypropylene were produced. The isotactic polypropylene comprised commercially available isotactic polypropylene, which is produced using a Ziegler-Natta catalyst. It is disclosed in the specification that the use of syndiotactic polypropylene widens the window of temperature over which thermal bonding can occur, and lowers the acceptable bonding temperature.
WO-A-96/23095 also discloses the production of fibres from blends including syndiotactic polypropylene which are either bi-component fibres or bi-constituent fibres. Bi-component fibres are fibres which have been produced from at least two polymers extruded from separate extruders and spun together to form one fibre. Bi-constituent fibres are produced from at least two polymers extruded from the same extruder as a blend. Both bi-component and bi-constituent fibres are disclosed as being used to improve the thermal bonding of Ziegler-Natta polypropylene in non-woven fabrics. In particular, a polymer with a lower melting point compared to the Ziegler-Natta isotactic polypropylene, for example polyethylene, random copolymers or terpolymers, is used as the outer part of the bi-component fibre or blended in the Ziegler-Natta polypropylene to form the bi-constituent fibre.
EP-A-0634505 discloses improved propylene polymer yarn and articles made therefrom in which for providing yarn capable of increased shrinkage syndiotactic polypropylene is blended with isotactic polypropylene with there being from 5 to 50 parts per weight of syndiotactic polypropylene. It is disclosed that the yarn has increased resiliency and shrinkage, particularly useful in pile fabric and carpeting. It is disclosed that the polypropylene blends display a lowering of the heat softening temperature and a broadening of the thermal response curve as measured by differential scanning calorimetry as a consequence of the presence of syndiotactic polypropylene.
U.S. Pat. No. 5,269,807 discloses a suture fabricated from syndiotactic polypropylene exhibiting a greater flexibility than a comparable suture manufactured from isotactic polypropylene. The syndiotactic polypropylene may be blended with, inter alia, isotactic polypropylene.
EP-A-0451743 discloses a method for moulding syndiotactic polypropylene in which the syndiotactic polypropylene may be blended with a small amount of a polypropylene having a substantially isotactic structure. It is disclosed that fibres may be formed from the polypropylene. It is also disclosed that the isotactic polypropylene is manufactured by the use of a catalyst comprising titanium trichloride and an organoaluminium compound, or titanium trichloride or titanium tetrachloride supported on magnesium halide and an organoaluminium compound, i.e. a Ziegler-Natta catalyst.
EP-A-0414047 discloses polypropylene fibres formed of blends of syndiotactic and isotactic polypropylene. The blend includes at least 50 parts by weight of the syndiotactic polypropylene and at most 50 parts by weight of the isotactic polypropylene. It is disclosed that the extrudability of the fibres is improved and the fibre stretching conditions are broadened.
It is further known to produce syndiotactic polypropylene using metallocene catalysts as has been disclosed for example in U.S. Pat. No. 4,892,851.
Recently, metallocene catalysts have also been employed to produce isotactic polypropylene. Isotactic polypropylene which has been produced using a metallocene catalyst is identified hereinafter as miPP. Fibres made of miPP exhibit much higher mechanical properties, mainly tenacity, than typical Ziegler-Natta polypropylene based fibres, hereinafter referred to as ZNPP fibres. However, this gain in tenacity is only partly transferred to non-woven fabrics which have been produced from the miPP fibres by thermal bonding. Indeed, fibres produced using miPP have a very narrow thermal bonding window, the window defining a range of thermal bonding temperatures through which, after thermal bonding of the fibres, the non-woven fabric exhibits the best mechanical properties. As a result, only a small number of the miPP fibres contribute to the mechanical properties of the non-woven fabric. Also, the quality of the thermal bond between adjacent miPP fibres is poor. Thus known miPP fibres have been found to be more difficult to thermally bond than ZNPP fibres, despite a lower melting point.
WO-A-97/10300 discloses polypropylene blend compositions wherein the blend may comprise from 25% to 75% by weight metallocene isotactic polypropylene and from 75 to 25% by weight Ziegler-Natta isotactic polypropylene copolymer. The specification is fundamentally directed to the production of films from such polypropylene blends.
U.S. Pat. No. 5,483,002 discloses propylene polymers having low-temperature impact strength containing a blend of one semi-crystalline propylene homopolymer with either a second semi-crystalline propylene homopolymer or a non-crystallising propylene homopolymer.
EP-A-0538749 discloses a propylene copolymer composition for production of films. The composition comprises a blend of two components, the first component comprising either a propylene homopolymer or a copolymer of propylene with ethylene or another alpha-olefin having a carbon number of 4 to 20 and the second component comprising a copolymer of propylene with ethylene and/or an alpha-olefin having a carbon number of 4 to 20.
It is known in the art to blend into a polypropylene produced using a Ziegler-Natta catalyst a second component comprising a random polypropylene, typically in an amount of around 20 to 50 wt % of the blend. Such a blend has been found to provide good thermal bonding when fibres produced from the blend are thermally bonded to form a non-woven fabric. The good thermal bonding results from a temperature overlap of the melting points of the Ziegler-Natta polypropylene and the random polypropylene. The thermal bonding is also achieved as a result of both the Ziegler-Natta polypropylene and the random polypropylene having relatively broad molecular weight distributions which provides a good blend and thus tends to enhance the thermal bondability of fibres.
It is an aim of the present invention to broaden the thermal bonding window of ZNPP fibres. It is a further aim of the invention to provide non-woven fabrics of ZNPP fibres exhibiting improved mechanical properties, in particular tenacity.
It is known that polypropylene fibres, and non-woven fabrics made of polypropylene fibres, tend to feel rough to the touch. It is also an aim of the present invention to improve the softness of polypropylene fibres.
The present invention provides a polypropylene fibre including greater than 50% by weight of a first isotactic polypropylene produced by a Ziegler-Natta catalyst, from 5 to less than 50% by weight of a second isotactic polypropylene produced by a metallocene catalyst and up to 15% by weight of a syndiotactic polypropylene (sPP).
The polymeric fibre may preferably include from 60 to 80% by weight of the first isotactic polypropylene and from 10 to less than 50%, more preferably from 20 to 40% by weight of the second isotactic polypropylene.
Preferably, up to lot by weight of the syndiotactic polypropylene (sPP) is included in the polypropylene fibre. The addition of sPP improves the softness of the fibres.
The first polypropylene produced by the Ziegler-Natta catalyst (ZNPP) may be a homopolymer, copolymer or terpolymer.
The second polypropylene produced by the metallocene catalyst (miPP) is a homopolymer, copolymer, being either a random or block copolymer, or terpolymer of isotactic polypropylene produced by a metallocene catalyst.
Preferably, the second polypropylene has a dispersion index (D) of from 1.8 to 8. Preferably, the second polypropylene has a melting temperature in the range of from 130 to 161xc2x0 C. for homopolymer and a melting temperature of from 80 to 160xc2x0 C. for a copolymer or terpolymer.
The miPP preferably has a melt flow index (MFI) of from 1 to 2500 g/10 mins. In this specification the MFI values are those determined using the procedure of ISO 1133 using a load of 2.16 kg at a temperature of 230xc2x0 C.
More preferably, the second polypropylene homopolymer or copolymer has an Mn of from 30,000 to 130,000 kDa and the MFI may range from 1 to 2000 g/10 min and preferably from 5 to 90 g/10 min for spunlaid or for staple fibres.
Preferably, the first polypropylene has a dispersion index (D) of from 3 to 12. Preferably, the first polypropylene has a melting temperature in the range of from 80 to 169xc2x0 C., more preferably a melting temperature of from 159 to 169xc2x0 C. for homopolymer and a melting temperature of from 80to 168xc2x0 C. for a copolymer or terpolymer. A typical melting temperature for ZNPP is 162xc2x0 C.
The ZNPP preferably has a melt flow index (MFI) of from 1 to 100 g/10 mins.
More preferably, the first polypropylene homopolymer has a MFI ranging from 15 to 60 g/10 min for spunlaid or 10 to 30 g/10 min for staple fibres
The sPP is preferably a homopolymer or a random copolymer with a RRRR of at least 70%. The sPP may alternatively be a block copolymer having a higher comonomer content, or a terpolymer. If the comonomer content is above 1.5 wt %, the sPP tends to become sticky, thus resulting in problems when spinning the fibres or thermally bonding the fibres. Preferably, the sPP has a melting temperature of up to about 130xc2x0 C. The sPP typically has two melting peaks, one being around 112xc2x0 C. and the other being around 128xc2x0 C. The sPP typically has an MFI of from 0.1 to 1000 g/10 min, more typically from 1 to 60 g/10 min. The sPP may have a monomodal or multimodal molecular weight distribution, and most preferably is a bimodal polymer in order to improve the processability of the sPP.
The present invention further provides a fabric produced from the polypropylene fibre of the invention.
The present invention yet further provides a product including that fabric, the product being selected from among others a filter, personal wipe, diaper, feminine hygiene product, incontinence product, wound dressing, bandage, surgical gown, surgical drape and protective cover.
The present invention is predicated on the discovery by the present inventor that when blended with a major amount of ZNPP, miPP causes improved thermal bonding of the ZNPP, without a significant modification of the mechanical properties of the fibres themselves. The present inventor has discovered surprisingly that by blending less than 50% by weight miPP into the Ziegler-Natta polypropylene, this provides enhanced thermal bonding of the Ziegler-Natta polypropylene despite the miPP having a narrower molecular weight distribution than that of the ZNPP, and also the random PP employed in the prior art referred to hereinabove, which would have been considered by the person skilled in the art to have reduced the thermal bonding effect.
Indeed, narrowing molecular weight distribution is known to reduce the bonding window temperature of the fibre. Thus the present inventor has discovered surprisingly that by blending of miPP into ZNPP, with the miPP having a typical melting range of from about 130xc2x0 C. to about 161xc2x0 C., which is lower than the typical melting range of ZNPP of from about 159xc2x0 C. to about 169xc2x0 C., the improvement in thermal bonding is achieved as a result of this lower melting point of the miPP, despite the narrower molecular weight distribution of the miPP which would suggest poorer thermal bonding. As a consequence, at any given thermal bonding temperature, more fibres are thermally bonded compared to pure Zn PP fibres and the bonding strength improves, thereby improving the mechanical properties of the non-woven fabric produced thereby.